1. Field of the Invention
This invention relates to the heat stabilization of polyamides against degradation at elevated temperatures, and more particularly to an improved process for the production of heat stabilized polyamide material.
2. Description of Related Art
Tensile, impact and dielectric properties of nylon degrade with time. The effect proceeds more quickly at elevated temperatures. A general mechanism of thermo-oxidative degradation in nylon includes the initiation and propagation of free radicals which react with oxygen to form a peroxy radical. This, in turn, abstracts a hydrogen atom from another polymer molecule to form a hydroperoxide and a second polymer radical. Hydroperoxides are unstable and decompose into two or more radicals and the cycle repeats inself.
One approach to inhibit thermo-oxidative degradation is to utilize chain-breaking antioxidant compounds to chemically capture free radicals and deactivate them, thus breaking the chain reaction in the propagation stage.
It is well recognized that copper in combination with a halide imparts thermo-oxidative stability to nylon and inhibits such degradation of properties. Apparently, copper is reduced by a halide to a cuprous state. At this lower oxidation state, the copper acts as an antioxidant to trap free radicals and is oxidized back to the cupric state. Excess halide can then reduce the copper back to cuprous state.
U.S. Pat. No. 2,705,227 to Stamatoff, issued Mar. 29, 1955, discloses heat stabilization of polyamides by incorporating copper in dissolved form into the polyamide along with a stability enhancing halogen compound. The reference is particularly concerned with nylon-6,6 and suggests the addition of the stabilizer composition to condensing reactants at any time during the condensation, preferably to the polyamide salt prior to condensation.
However, such teachings of manner of addition of stabilizer compositions are not relevant to the nylon-6 process. Nylon-6 is produced by the polymerization of caprolactam. The caprolactam is heated in the presence of water and catalysts. Hydrolysis takes place, the ring opens, and polymerization occurs by an addition reaction. In a polycondensation step, linking of polymer chains to the appropriate molecular weight is effected by the removal of water. Molecular weight is controlled by use of chain terminators or the duration of the polymerization step. Under these conditions polymerization does not go to completion but to an equilibrium between monomer and polymer. At this stage and in molten form, the polymer-monomer mixture is discharged through multistrand dies and is pelletized or formed into chips. The monomer and low molecular weight fractions or oligomers are removed from the pellets by extraction or leaching with hot water, after which step the pellets are dried. During this leaching step, a noticeable quantity of copper and a major proportion of the halogen compounds would be lost if they were added prior to extraction. The preferred halides such as KI, NaI, and NaBr, are highly water soluble and thus prone to loss during extraction.
Impregnation of the desired additives to the leached chips has become an acceptable method of introducing the stabilizer compounds. Copper, iodide, bromide and other additives such as magnesium can be impregnated from an aqueous solution. For impregnation the pellets are introduced into a vessel where they reside for a time, typically a few hours, while impregnation fluid is circulated through the vessel. There are major problems resulting from an impregnation process however. Formation of precipitates is inevitable due to presence of reactive components. The formation of precipitates results in clogging-up solution filters. For example, Cu(OH).sub.2, CuIOH, Cu(OH)(CH.sub.3 COO) and other insoluble salts could be formed depending on temperature, pH and concentration. The precipitates also affect the color of the chips in an undesirable manner and reduce the efficiency of the process apparently due to loss of additives. The process is difficult to control, requiring one to three days to reach steady state after the beginning of the impregnation. Continuous monitoring and control of numerous variables are essential, with unexpected upsets the rule rather than the exception.
The method of the present invention addresses these shortcomings in an efficient, simple manner and produces product with acceptable properties.